Re: [PATCH 1/2]middle-end Fold BIT_FIELD_REF and Shifts into BIT_FIELD_REFs alone

[Andrew Pinski <pinskia@gmail.com>]

On Fri, Sep 23, 2022 at 4:43 AM Tamar Christina via Gcc-patches
<gcc-patches@gcc.gnu.org> wrote:
>
> Hi All,
>
> This adds a match.pd rule that can fold right shifts and bit_field_refs of
> integers into just a bit_field_ref by adjusting the offset and the size of the
> extract and adds an extend to the previous size.
>
> Concretely turns:
>
> #include <arm_neon.h>
>
> unsigned int foor (uint32x4_t x)
> {
>     return x[1] >> 16;
> }
>
> which used to generate:
>
>   _1 = BIT_FIELD_REF <x_2(D), 32, 32>;
>   _3 = _1 >> 16;
>
> into
>
>   _4 = BIT_FIELD_REF <x_1(D), 16, 48>;
>   _2 = (unsigned int) _4;
>
> I currently limit the rewrite to only doing it if the resulting extract is in
> a mode the target supports. i.e. it won't rewrite it to extract say 13-bits
> because I worry that for targets that won't have a bitfield extract instruction
> this may be a de-optimization.

It is only a de-optimization for the following case:
* vector extraction

All other cases should be handled correctly in the middle-end when
expanding to RTL because they need to be handled for bit-fields
anyways.
Plus SIGN_EXTRACT and ZERO_EXTRACT would be used in the integer case
for the RTL.
Getting SIGN_EXTRACT/ZERO_EXTRACT early on in the RTL is better than
waiting until combine really.


>
> Bootstrapped Regtested on aarch64-none-linux-gnu, x86_64-pc-linux-gnu
> and no issues.
>
> Testcase are added in patch 2/2.
>
> Ok for master?
>
> Thanks,
> Tamar
>
> gcc/ChangeLog:
>
>         * match.pd: Add bitfield and shift folding.
>
> --- inline copy of patch --
> diff --git a/gcc/match.pd b/gcc/match.pd
> index 1d407414bee278c64c00d425d9f025c1c58d853d..b225d36dc758f1581502c8d03761544bfd499c01 100644
> --- a/gcc/match.pd
> +++ b/gcc/match.pd
> @@ -7245,6 +7245,23 @@ DEFINE_INT_AND_FLOAT_ROUND_FN (RINT)
>        && ANY_INTEGRAL_TYPE_P (type) && ANY_INTEGRAL_TYPE_P (TREE_TYPE(@0)))
>    (IFN_REDUC_PLUS_WIDEN @0)))
>
> +/* Canonicalize BIT_FIELD_REFS and shifts to BIT_FIELD_REFS.  */
> +(for shift (rshift)
> +     op (plus)
> + (simplify
> +  (shift (BIT_FIELD_REF @0 @1 @2) integer_pow2p@3)
> +  (if (INTEGRAL_TYPE_P (type))
> +   (with { /* Can't use wide-int here as the precision differs between
> +             @1 and @3.  */
> +          unsigned HOST_WIDE_INT size = tree_to_uhwi (@1);
> +          unsigned HOST_WIDE_INT shiftc = tree_to_uhwi (@3);
> +          unsigned HOST_WIDE_INT newsize = size - shiftc;
> +          tree nsize = wide_int_to_tree (bitsizetype, newsize);
> +          tree ntype
> +            = build_nonstandard_integer_type (newsize, 1); }

Maybe use `build_nonstandard_integer_type (newsize, /* unsignedp = */ true);`
or better yet `build_nonstandard_integer_type (newsize, UNSIGNED);`

I had started to convert some of the unsignedp into enum signop but I
never finished or submitted the patch.

Thanks,
Andrew Pinski


> +    (if (ntype)
> +     (convert:type (BIT_FIELD_REF:ntype @0 { nsize; } (op @2 @3))))))))
> +
>  (simplify
>   (BIT_FIELD_REF (BIT_FIELD_REF @0 @1 @2) @3 @4)
>   (BIT_FIELD_REF @0 @3 { const_binop (PLUS_EXPR, bitsizetype, @2, @4); }))
>
>
>
>
> --